1. Field of the Invention
The present invention relates to a multi-range, synchronously shiftable, regenerative, continuously variable hydromechanical transmission. More particularly, the invention relates to a multi-range, synchronously shiftable, regenerative, continuously variable hydromechanical transmission suitable for application to high speed engines.
2. Description of the Related Art
Over the years, many vehicle drivetrains have been designed to utilize hydraulic transmission for multiplying engine torque to accelerate a vehicle from rest to a desired maximum speed. However, such design efforts have typically resulted in hydraulic transmissions that, although having acceptable torque capacity, are undesirably large and heavy. Moreover, such transmissions have exhibited less than optimum efficiency, resulting in loss of fuel economy and/or transmission performance.
As an alternative to the purely hydraulic transmissions, hydromechanical transmissions have been utilized in drivetrains for large off-road construction and military vehicles. Such transmissions are typically of a split power input type. In the split power input type transmission, a hydrostatic power unit (HSU) and a mechanical power unit are driven in parallel by a vehicle engine. The HSU converts its split portion of input power from the engine into hydrostatic output power that can be infinitely varied in speed and torque over a particular hydrostatic stroke range. This hydrostatic output power is then combined with the split portion of engine power in the mechanical power unit to produce hydromechanical output power in multiple transmission ranges. The speed and torque in each of the transmission ranges, which are initially set by gear ratios of the mechanical power unit, can be infinitely varied by varying the stroke of the HSU.
A properly designed hydromechanical transmission can advantageously provide synchronous range shifting that affords smooth and uninterrupted power flow from engine to driving wheels, as the vehicle is accelerated from rest to maximum speed. An additional benefit is that the engine may be operated at or near its peak efficiency output speed, regardless of transmission output speed.
However, there are several limitations in utilizing the HSU for vehicle transmissions. One limitation is the input speed capacity of the HSU with respect to its output torque potential. Speed capacity (RPM) is limited by high centrifugal loading on certain internal components of the HSU and the ability to supply replenishment hydraulic fluid to replace losses due to leakage. This limitation is of less concern for relatively low speed engine applications, such as diesel engines. Diesel engines generally have the maximum engine speed of less than 2600 rpm. Thus, the HSU can be directly driven by a diesel engine. On the other hand, maximum speeds of gasoline engines can, for example, reach 6000 rpm. Therefore, when the HSU is coupled with such a high speed engine, the input speed of the HSU must be reduced, i.e., geared down, to a speed that is safely acceptable for the HSU.
However, gearing down the input speed of the HSU has undesirable effect of reducing the output speed of the HSU and thus the overall ratio range of the transmission itself. That is, if the input speed of the HSU is reduced to 50% of the engine speed, then the maximum output speed of the HSU is also reduced to 50% at 1:1 input-output speed ratio. Moreover, because torque is inversely proportional to speed, a HSU with a larger capacity is necessary to accommodate the same engine power.
While high output torque is required to start a vehicle in motion from rest, fortunately, torque requirements decrease as vehicle speed increases. Therefore, it is desirable to provide a high speed driven hydromechanical transmission that takes advantage of both the higher starting torque capacity of a geared-down HSU and the increased output speed of a geared-up HSU.